Recently, an electronic endoscope having a function as a spectrometer has been proposed as described, for example, in Japanese Patent Provisional Publication No. JP2007-135989A. By using such an electronic endoscope, it is possible to obtain the spectral property (the distribution of light absorption property for each frequency) of a living tissue such as a mucous membrane of a digestive organ, e.g., a stomach or a rectum. It is known that the spectral property of a substance reflects information concerning the types or densities of components contained in the vicinity of a surface layer of a living tissue being an observation target, which is established in a field belonging to academic frameworks of analytical chemistry. It is also known in this field that the spectral property of a substance consisting of a composite is information obtained by superimposing the spectral properties of essential components constituting the composite.
A diseased portion in a living tissue may contain a substance having a chemical configuration that is rarely contained in a healthy living tissue. Therefore, a spectral property of a living tissue containing a diseased portion is different from a spectral property of a living tissue containing only a healthy portion. Since the spectral properties of the healthy portion and the diseased portion are different from each other as described above, it becomes possible to judge whether or not a living tissue contains a diseased portion by comparing the spectral property of the healthy portion with that of the diseased portion.